Wave transmission system



l 1,696,315- R. c. MATHEs y uva' TRANsxIssIoN SYSTEM y Vrigina11='i1edNov.1, 1924 Patented Dec. 2.5, i i

jugmpz-gsfr.

noBEET c. MATEES, ,or WYOMING,

COMPANMICOEPORATED, or imw'sromr,4 N. Y.,

vNEW ,'rEns'Er, IrissIefNon To WESTERN lEnrofrm'c- A CORPORATION F NEW YORK.

- wAvE TRANSMISSION SYSTEM.

:Originali application filed Novemberl, 1924; Seriali-'0. 747,164; .Divided and this application iled y December 16, 1926.'

This ,inventionfl relates to 'wave transmission systems and particularlyto systems re- `quiring delayed wave transmission for their proper functioning andis a division o pat ent application` Serial No. 7417 ,164, liled November 1, 1924. f@

- An object .ofV this invention is the improvement opftr'ansmission systems utili-zing, delay circuits or'netvvorks.J 5 g In transmission systems-of the type reterred to, such asvoice operatedr repeater cir cuits and echo suppressing,-arrangements, it has' beenvfound advantageous to provide a delay in the ytransmission ot' the waves dur: ing the time required for the relays or other circuit controlling devices to operate. i Heretofore, such a delay has `'been obtained bythe use of electrical networks, artiicial loaded lines? In accordance-with this yinvention a mechanical transmitting means is provided having mass andelasticity so proportioned as to obtain a required time delay.

In Aone embodiment of the invention a inechanical transmission path` of distributed constants, preferablyy an acoustical system is employed in an electrical circuit for obtain inga required time delay, the electrical waves being converted into acoustical wares at one end of this system and the acoustical waves\ being converted into electrical waves at the other. A' vIt has been found that in most electrical systems where any considerable .delay is re- -quired the cost of amechanical delay system including the arrangements for convert-ing the electrical waves to mechanical Waves and back to electrical waves is considerably less than the. cost of an .electrical network, since .the ordinary type of electrical netwcrlcre-y quires alarge number of sections which `are expensive tor construct. l K

Delay" systems are 'defined sutliciently by the statement of theirrunction. They must delay the transmission of waves bye required amount rand in so doing must not have' any other effect -or atleast no detrimental effect on the transmitted Waves, although particular designs of delay circuits may, for example, be

such as iilters or i serial Np. 155,155.

used to give a delay which is diierent for the so different frequencies in .a transmitted band.

'to compensate a delay characteristic already presentdn the transmission Such a delay circuit 1s disclosed and claimed in application of SilenlqSerial No. 64,882, led October L 2li, 1925. A time delay isv fundamentally different from a mere phase shift in that the time of delay is large compared to the period of thewaves transmitted. f

The characteristics Gif-:wave motion are analogous in mechanical and electrical systems so that the fundamental mechanical and electrical equations are identical except for the symbols employed. The corresponding quantities in the two systems are:

VMechanical systems. `lllectrcalsystems.

Force. i n Electromotive force. Displacement. y Charge. i Velocity of vibration. Current. 10 Mass. Inductnnce. Strness or elasticity. Reciprocal of capacity. Fr1ct1on. y Resistance. .K

The transmission characteristic of any system for the propagation ot wave energy is measured in terms of the propagation constant P. ln an electrical system the propaga-- tion constant per unit length of a uniform line, or per section or a line ofrecurrent structure is defined as the natural" logarithm g() of the vector ratio of the steady.' state cur- `rents at two points separated by a unit length in a'unirorm line or' infinite length or at'two successive corresponding points in a line of recurrent structure or infinite length, that is,

i .P =logel1 2,

`part A is calledthe attenuation const-ant and' thefimaginary part B is called the phase or wave length constant, i. e., the retardation angle in radians. The velocity (V) of the propagation of a Wave of angular velocity (tu) S (AJ V=13 and the time of transmission or delay (t) per unit length or per section 1s This invention can be morereadily understood by reference to the following detailed description in connection with the ,lrawing in which Fig. l shows an acoustical delay system according to this invention. Fig. shows a mechanical system of lumped constants, Fig.` 3 shows an electrical system equivalent to the mechanical system at' ig. 2, and Fig. 4 shows a four-wire combined land line and radio system to which this in-' vention isapplicable.

Referring to Fig. l which shows/an acoustical delay circuit, incoming electrical waves arel converted into acoustical waves by means of the electromagnetic receiver R and the delayed mechanical waves reconverted into electrical waves by means of the transmitter T. Intermediate transmission is by means of acoustic Waves directed and regulated by the sound conduit C in accordance with conventional practice in the construction of the horn elements in acoustic transmitters and receivers. The delay in this type oitransmitting medium is inherentin -the material of the medium, although if the elements of this medium were reduced to molecular dimensions it would be obvious that the conditions inherent in the operation of a system employing lumped constants, as shown in Fig. 2, are closel paralleled. In common in the two types o systems,.transmission is effected by ,wave motion, the velocity of propagation being determined by the mass (o r density) and the' stiffness (or elasticity) characteristics of the medium. In the arrangement of Fig. 1 the desired delay is regulated solely .by varying the length of the path since the characteristics of the medium are fixed.-

The design of this type of system is parallel to that of an electrical line in which there isno leakage. Ifthe attenuation due to friction Ais small the delay may be determined as though the waves were propagated in a dissipationless (free) medium. iHowever, if this attenuation is large the velocity of propagation will be considerably lower and may to some extent depend upon the frequency of the transmitted waves.

Fig. 2 illustrates a type of mechanical delay circuit employing lumped constants. This system is analogous to the electrical system of Fig. 3, the mass of elements M and the stiffness of elements S being analogous, respectively, to the inductance of the coils and the capacity of the condensers of Fig. 4. The design data pertaining to electrical delay circuits o f this type, as discussed in an article entitled Physical theory of the electrical wave filter by G. A. Campbell, in the Bell System Technical Journal for November, 1922, are equally applicable to the mechanical delay circuit of Fig. 2, having regard for the differences in units and conversion factors. For ideal conditions simulating those existing in an electrical filter circuit niade up of lumped inductanccs and capacities each of which is completely individual in its function. the masses must be assumed to be perfectly rigid, the stillness elements, springs, lfor example must be assumed to have no weight, and pendant means I must be assumed to be perfectly flexible. Conversion from electrical to mechanical energy and back to electrical energy is accomplished by means of the receiver element R and the transmitter element T, as in the system of Fig. l. The first'. mass element at the left is driven by the receiver diaphragm. In a manner entirely analogous to the operation of the end section of the network of Fig. 3, the energy is absorbed in actuating the mass into movement. After an interval determined by the time constant of the first section, comprising a mass and a stiffness element, this energy will have been stored in the mass and then expended in storing up an 'equivalent amount of energy in the first stifness element. This stifl'ness element, the first spring, in turn discharges this energy into the second mass element, and so on throughout the transmission path. It is substantially immaterial so far as the physical principle is concerned whether theJnechanical energy transfers occur in the single plane including the several pendant means, or in planes transverse thereto. The number of sections, in a practical case, would be many times as great as that shown.

Fig. -l shows an electricalsystem in which the delay circuit ot' this invention may be advantageously employed. Referring to this ligure, L1 and L2 indicate low frequency linesconnected, for example, to cent-ral telephone. exchanges. In the particular circuit that will be assumed for convenience, L, is connected to a central station in New York city and L2 to a central station in London. The circuits between these lines are adapted to transmit signals in both directions over individual paths, partly by wire and partly by radio. The radio link comprises the portions between antennae AN1 and AN2 for the east branch of the four-wire circuit, and between antenna: A N, and AN, for the west branch of the four-wire circuit. Antenna: AN, and AN3 in the practical embodiment assumed are located at or near Rocky Point, Long Island and antennze AN2 and AN4 are located at London, England. The circuits at the east lua lll.)

, station connecting the'y antennae vvith the low' j frequency line comprise, 1n the receiving portion, the' receiving circuit RGir and amplifier AM1,` [the corresponding ycircuits yfor the transmitting portion comprising transmitter A T1 and amplilier AM2. These, transmitting and vreceiving portions kofthe-circuit are joined to the lon7 frequency'line L2 by means 'of a bridge transformer yHl Which cooperates 4with line L2 and with 'balancing network N1 in a Well-known manner rto insure conjugacy4 between thetransmitting yand receiving pori y tions. The vamplifiers AM1 and AM2 may be reo.

\ lated high frequency Wave to reproduce the .used or not as desiredgexpediency'vvill justify their use in the usual practical case. The transmitter T1 and vreceiver RC1' embody combinations of conventional elements for respectively generating andl transmitting a signal modulated higlrfrequency Wave and for demodulating an incomingsignal modusignal. The antennae AN 2 and ANdjare geef` graphically or Ageometrically related to each other in 'such manner .that local singingat that station is prevented.,` rlhis effect may beaided'by ythe use oi differenty frequencies for thetvvo directions of transmission and the irequencysel'ection at the antennae made possible thereby. n i f v The circuitsat the West station betweenthe antennae AN l. andAN3 andthe low frequency line L1 comprise av low, frequency transmis# sion ypath of several milesA in; length; The

transmittinga'nd receiving' portions of this path are conjugately relatedsto lovv frequency line L1 by the hybrid 'coil arrangement itl2 f and balancing.network` N2, whichl function in the saine manner as the similar elements at theeast station.v Transmitter T2 and rek ceiverRC2 are similar to, andfunction in the Videntical manner as the respective devices T1 and RC1. The low' frequency transmission circuit atthis station comprises the ampliiiersAlfI2 and AM'4iand`the ldelay lcircuit D. This'delay circuit may beef the 'mechanical type illustrated in Figs. lor 2.l 'lhe' criteria for the efficient accomplishment of thisffunc 'K tion will be describedlater. The transmis?. f sioncircuit for-receiving at this station may Vsimilarly comprise theampliiiers'AM5 and AMG. Antennae AN 1 and ANs are so related to each other, by any ofthe means referred to in thedesoription of the east station cir` cuits, as to prevent singing at'the v'veststa.-Y

a tion. For example, .one or lboth of these. anv tenn may be made directive in such a direc tion as ,tof exclude reaction on or from the otherantenna. As a'. practical ,matter it' will also be rfound expedient to positionthese antennae afmil'e or more apart. 'AmplifiersAM and AM may conveniently be located at their respective* antennae. Amplifiers and' AM., may together V constitute a four-wire repeativng station located intermediatethe antennse andthe low -IineLla f tive.

actuated to open circuit position by relay R2;

Foreliicient operation of such a four-Wire'- circuit it is desirable that only that branch be operative for transmission which is being employed at the moment. This insures that the round-trip7 path for singing or echo currents is vat all times interrupted. The

means for' actuating the switches l and `2`so to satisfy this requirement will now loe-described. v l Y i #During transmission east, signal currents How throughampliers AM3 and AM4 and thence through delay circuit D to theradio transmitter. A relay'control circuit 3 is d eim Signal v currents flowing rived from this branch at theinput of the delayv circuit. through this vcontrol circuit vare amplified and rectified by device AMR and the resultant cur-rents iiovv through and actuate relays R1 and R2. lli-elay R1 actuates switch 1 to circuit closing position and accordingly renders the West'branch inoperative. Relay R2 Vactuates switch 2 to open circuit position and accordingly renders the east branch opera The delay circuit D insures that'the actufation of switch 1 Will have been completed before echo currents transmitted to'and returned from the east station have progressed to thepoints of connection of the circuit containing 'switch l. This delay circuit alsoinlsures that the switchQ Will have been actuated to render the east branch operative before the initial signal impulses reach the output of the delay circuit, so that clipping is prevented.l Since it required less time to openV svvitch than it does to close switch l, the designpof a delay circuit to satisfy the first' condition 'would ordinarily be effective for satisfying the second condition. This 4obviously would not be ytrue if' the West banchj Were made inoperative by a circuit opening operation, as Wouldbe possible by a simple rearrangement ot thecircuits. However, it has been found eX edient in a practical case to use the circuit c osing alter natiire disclosed.- The` amplifier-rectifier AMB rmay comprise any means for operating on the signal currents to obtain a wave which can yeliciently operate relays R1 and R2.' In a practical caseiit would comprise a combination oi' .ampliiie'r and'. rectifier elements, preferably of the vacuum tube type. An ex ample`- of an arrangement Welladapted for the purpose,y is disclosed an applicationy of Hartley, Serial No.' 602,273, filed November 20, 1922. The switches 1 and 2 are biased,

. by means not shown, so 'as to return respectively to their operi and closed positionswhen not actuated by the relays R, and R2.

During west transmission the detected signal .currents will find an uninterrupted path to the low frequency line L1. Since the east organize the circuits for eastward transmission and accordingly produce a periodical vibration of the switches 1 and 2 with an attendantI interruption of the received signals and .e tendency to singing and echo phenomena, a portion of the received signal currents interrupts this relay control circuit by opening the switclrfi. This is accomplished by relay R3 in relay control circuit 5. Ampli- Iier rectifier AMR, in this circuit has the same function as that of AMR in circuit 3. The delayI in transmission over the land line from the output of amplifier AM6 to the output of amplifier AM4 insures that switch 4 will be opened before these currents reach the out put of amplifier AM. What is claimed is: 1. In a Wave transmission system which requires for its proper functioning a delay in the transmission otl Waves at a point in the system, large compared with the period of the waves, delay means comprising a mechanical line, the mass and elastic properties ot which, together with the length of the line, are proportioned 4to give the desired delay, said line comprising a homogeneous medium having the nass and elastic properties continuously distributed along a substantial portion of its length. i

2. `lVave delaying means of the type and for the purpose described in claim 1, in which the mechanical line comprises a gaseous nicdium enclosed in a sound conduit.

3. In an electrical wave transmission system which requires for its proper functioning a time delay in the wave transmission at a point in the system, said required delay being large compared with the period of the transmitted waves, delay means comprising an enclosed air column, the mass and elastic properties of which, together with the length, are proportioned to give the desired delay, and electro-mechanical coupling m'ans for connecting said air column between two portions of the electrical system.

4. lVave delaying means of the type and for the purposedeseribed in claim 3, in which the electromechanical coupling means comprises a telephone receiving instrument at one end of the column and a telephone transmitter at the other end.

In Witness whereof, I hereunto subscribe my name this 14th day of December, A. D. 1926.

ROBERT C. MATHES. 

